Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

A photovoltaic cell for use in a solar cell panel and a method of forming
a photovoltaic cell for use in a solar cell panel are disclosed. The
photovoltaic cell includes a plurality of first layers of a first
material having a first thickness and a first optical characteristic; a
plurality of second layers of a second material having a second thickness
and a second optical characteristic, each of the plurality of layers of
the first material adjacent to two of the plurality of layers of the
second material; wherein the second material includes a metal. In one
aspect, the first material includes a semiconductor. In a further aspect,
the plurality of first layers includes layers formed from two different
semiconductor materials.

Claims:

1. A photovoltaic cell for use in a solar cell panel comprising: a
plurality of first layers of a first material having a first thickness
and a first optical characteristic; a plurality of second layers of a
second material having a second thickness and a second optical
characteristic, each of the plurality of layers of the first material
adjacent to two of the plurality of layers of the second material; and
wherein the first material is selected to absorb light in the visible
spectral region; and the second material comprises a metal that is
coupled to conduct a current.

2. The photovoltaic cell of claim 1 wherein the plurality of layers of a
first material comprises a semiconductor material.

3. The photovoltaic cell of claim 2 wherein the plurality of layers of a
first material includes a first semiconductor material forming at least
one layer and a second semiconductor material, different than said first
semiconductor material, forming an additional layer.

4. The photovoltaic cell of claim 1 wherein a thickness of the cell is
less than about one micrometer.

5. The photovoltaic cell of claim 1 wherein the plurality of layers of a
second material comprises an electrode network.

6. The photovoltaic cell of claim 1 wherein the first and second optical
characteristics comprise the indices of refraction of the first and the
second materials, respectively.

7. The photovoltaic cell of claim 1 wherein the first material is
selected from the group consisting of a semiconductor material and a
doped dielectric material.

8. The photovoltaic cell of claim 1 wherein the plurality of layers of a
first material comprises layers of a third material and layers of a
fourth material; wherein: the third material is substantially transparent
to light in the near infrared spectral region and absorbs light in the
blue portion of the visible spectral region; and the fourth material
absorbs light in the near infrared spectral region.

9. The photovoltaic cell of claim 6 wherein the layers of the third
material are closer to a side of the photovoltaic cell exposed to the
sun, and the layers of the fourth material are closer to an opposite side
of the photovoltaic cell.

10. The photovoltaic cell of claim 2 wherein an average light absorption
across the visible spectral region is higher than about 60% with a
corresponding average absorption in semiconductor layers of approximately
40%.

11. The photovoltaic cell of claim 2 wherein an average light absorption
across the visible spectral region is about 90% with a corresponding
average absorption in semiconductor layers of approximately 70%.

12. The photovoltaic cell of claim 1 wherein the second material is
substantially opaque for wavelengths of light in the visible spectrum.

13. The photovoltaic cell of claim 1 wherein the plurality of layers of
the first material and the plurality of layers of the second material
form a photonic bandgap structure having high reflectivity in the
ultraviolet (UV) and infrared (IR) wavelength spectral range.

14. The photovoltaic cell of claim 1 wherein the plurality of layers of a
first material comprises layers of a plurality of materials; wherein: the
plurality of materials comprises semiconductor materials having
electronic bandgaps between about 0.5 eV and about 4 eV.

15. A method of forming a photovoltaic cell for use in a solar cell panel
comprising: forming a plurality of layers of a first material having a
first thickness and a first optical characteristic; forming a plurality
of layers of a second material having a second thickness and a second
optical characteristic, such that each of the plurality of layers of the
first material is adjacent to two of the plurality of layers of the first
material; wherein: the first material is selected to absorb light in the
visible spectral region; and the second material comprises a metal and is
coupled to conduct a current.

16. A photovoltaic film, comprising: a body formed of at least one
semiconductor material intercalated with at least three metal layers, at
least two of the metal layers electrically connected to each other;
wherein the body has a thickness of less than 700 nm and the plurality of
metal layers have a combined thickness less than 200 nm.

17. The film of claim 16, the body includes a first semiconductor
material substantially transparent to light in the near infrared spectral
region and absorbs light in the blue portion of the visible spectral
region; and a second semiconductor material that absorbs light in the
near infrared spectral region.

18. The film of claim 17, wherein the first semiconductor material is
formed in a layer having a first thickness and the second semiconductor
material is formed in a layer having a second thickness, the second
thickness being at least twice as large as the first thickness.

19. The film of claim 16, wherein the semiconductor material and at least
three metal layers are arranged to absorb at least 40% of the light
across the visible spectral region and reflects greater than 50% of the
light across the infrared spectral region.

20. The film of claim 19, wherein the semiconductor material and at least
three metal layers are arranged to transmit greater than 25% of the light
in the visible spectral region.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application relates to and claims priority to U.S. Provisional
Patent Application No. 61/499,915 filed Jun. 22, 2011, entitled "Photonic
Bandgap Solar Cells," the disclosure of which is incorporated by
reference, in its entirety here for all purposes.

BACKGROUND OF THE INVENTION

[0003] Embodiments described in the present disclosure relate generally to
the field of solar cells and conversion of light energy into electrical
energy.

[0004] In the field of solar panel cells it is desired that solar light be
absorbed efficiently by a material, and converted into charge carriers
that may generate an electrical current (photocurrent). Solar cells use a
photon absorption process where an incoming photon generates charge
carriers such as an electron-hole pair in the material. The
photo-generated electron-hole pair may be converted into a photocurrent
by applying an electric field separating the charge carriers in
conducting elements. This is a desirable outcome of a photo-generated
electron-hole pair in a solar cell. Alternatively, the photo-generated
electron-hole pair may recombine, emitting a second photon at the same or
slightly different wavelength. The second photon may then escape the
structure. Also, the photon-generated electron-hole pair may be trapped
in the material by impurities and other defects, without generating a
photocurrent. Further, scattering events in impurities and other defects
may deplete the energy of the photo-generated electron-hole pairs, so
that these may recombine, generating excess heat and unable to produce a
photocurrent.

[0005] Current solar panel cell applications rely on amorphous silicon or
similar bulk structures in order to optimize the conversion of absorbed
photons into a photocurrent. Amorphous structures facilitate the
re-absorption of photons that are re-emitted within the structure,
increasing the charge carrier generation. However, amorphous materials
have the problem of inefficient coupling of the generated charge carriers
into a photocurrent, through an electric field. Thus, prior art
applications of solar cells use complicated structures involving highly
doped semiconductor regions next to Schottky barrier metals. Furthermore,
in order to increase the amount of generated charge carriers, some
technologies choose to use thicker slabs of materials. This increases the
probability of trapping charge carriers in material defects before being
coupled to a current flow out of the structure, thereby reducing
photocurrent generation efficiency.

[0006] Current solar panel technologies present problems such as wear out,
damage, and stress introduced in the structure by heating. Damage and
heating in a solar panel is produced by absorption of the high content of
ultraviolet (UV) and infrared radiation (IR) from the sun. Also, solar
panel technologies face the problem that the efficiency of the
photocurrent generation is highly dependent on the angle of incidence of
radiation. The sun provides radiation at variable angular orientations
throughout the day. Thus, current technologies need to devise complicated
mechanisms and architectures to compensate for the change in efficiency
throughout the day.

[0007] Therefore, there is a need for solar panel cells that have
increased efficiency for photocurrent generation. It is desirable that UV
and IR radiation either be reflected by the solar panel cells to avoid
damage to the structure or absorbed in materials having the appropriate
band gaps to convert the radiation in photocurrent. It is also desired
that the efficiency of the current generation process be equally high for
all possible incidence angles of the radiation upon the solar panel.
Current solar panel technology uses tracking devices to overcome this
issue.

SUMMARY

[0008] According to embodiments disclosed herein a photovoltaic cell for
use in a solar cell panel includes a plurality of layers of a first
material having a first thickness and a first optical characteristic; a
plurality of layers of a second material having a second thickness and a
second optical characteristic, each of the plurality of layers of the
first material adjacent to two of the plurality of layers of the second
material; wherein the second material includes a metal. In one form, the
first material is a semiconductor. In a further aspect, the type of metal
remains the same for each layer of the second material, while the first
material include more than one semiconductor with a first semiconductor
material used to form at least one layer and a different semiconductor
material used to form a different layer.

[0009] Further according to embodiments disclosed herein a method of
forming a photovoltaic cell for use in a solar cell panel includes
forming a plurality of layers of a first material having a first
thickness and a first optical characteristic; forming a plurality of
layers of a second material having a second thickness and a second
optical characteristic, such that each of the plurality of layers of the
first material is adjacent to two of the plurality of layers of the first
material; wherein the second material includes a metal.

[0010] These and other embodiments are further described below with
reference to the following figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1A shows a partial view of a photonic bandgap solar cell
according to some embodiments.

[0012] FIG. 1B shows the transmission, reflection, and absorption of light
at normal incidence for a photonic bandgap solar cell according to some
embodiments.

[0013]FIG. 2A shows the transmission of p-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell
according to some embodiments.

[0014] FIG. 2B shows the reflection of p-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell
according to some embodiments.

[0015]FIG. 2c shows the absorption of p-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell
according to some embodiments.

[0016] FIG. 3A shows the transmission of s-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell
according to some embodiments.

[0017] FIG. 3B shows the reflection of s-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell
according to some embodiments.

[0018]FIG. 3c shows the absorption of s-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell
according to some embodiments.

[0019] FIG. 4A shows a partial view of a photonic bandgap solar cell
according to some embodiments.

[0020]FIG. 4B shows the transmission, reflection, and absorption of light
at normal incidence for a photonic gap solar cell, according to some
embodiments.

[0021] FIG. 5A shows the transmission of p-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell
according to some embodiments.

[0022]FIG. 5B shows the reflection of p-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell
according to some embodiments.

[0023]FIG. 5c shows the absorption of p-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell
according to some embodiments.

[0024]FIG. 6A shows the transmission of s-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell
according to some embodiments.

[0025]FIG. 6B shows the reflection of s-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell
according to some embodiments.

[0026]FIG. 6c shows the absorption of s-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell
according to some embodiments.

[0027] FIG. 7A shows a partial view of a photonic bandgap solar cell
according to some embodiments.

[0028] FIG. 7B shows the transmission, reflection, and absorption of light
at normal incidence for a photonic bandgap solar cell according to some
embodiments.

[0029] FIG. 8 shows a partial view of an electrical connection for a
photonic bandgap solar cell according to some embodiments.

[0030] FIG. 9 shows a partial view of an electrical connection for a
photonic bandgap solar cell according to some embodiments.

DETAILED DESCRIPTION

[0031] A photovoltaic cell to convert light energy into electrical energy
according to embodiments disclosed herein includes a juxtaposition of
metallic and properly doped semiconductor layers. As illustrated in some
embodiments, relatively thin dielectric layers may be embedded between a
metal layer and a semiconductor layer to increase the open circuit
voltage (Voc). The resulting multilayer stack absorbs the part of
electromagnetic spectrum that is efficiently converted into electric
energy by the semiconductor layers and, at the same time, reflects back
the other parts of the spectrum that may cause overheating of the cell
(for example near infrared radiation) and are generally detrimental to
the overall conversion efficiency. The geometrical parameters of the
multilayer, as well as the optical properties of the materials composing
the stack, can be used to control absorption, reflection, and
transmission spectra using design criteria similar to those exploited in
photonic band gap (PBG) structures. The metallic layers play a two-fold
role in device operation: (i) from an optical point of view the metal
regions act as mirrors, thus providing multiple reflections, slow light
effects, field amplitude enhancement, and enhancement of absorption of
electromagnetic energy propagating through the structure; (ii) from an
electrical point of view, the metallic layers can be used as a network of
electrodes distributed along the entire structure.

[0032] FIG. 1A shows a partial view of a photonic bandgap solar cell
according to some embodiments. In the embodiment depicted in FIG. 1A, a
semitransparent metal-semiconductor solar cell is depicted. The
multilayer structure of FIG. 1A includes a combination of properly doped
semiconductor layers made of GaAs and GaP having thicknesses of 20 nm and
40 nm, intercalated with thin metal layers of silver (Ag), of 15 nm
thickness each. FIG. 1A includes a first layer of GaP having a 20 nm
thickness placed on top of two more layers of GaP of 40 nm thickness.
Four layers of GaP, each having a thickness of 40 nm, are placed below
the GaAs layers, and a 20 nm GaAs layer is at the bottom of the
structure. Each of the semiconductor layers are separated from each other
by a 15 nm layer of Ag. The structure shown in FIG. 1A has seven (7)
layers of Ag, thus having a total thickness of 105 nm of Ag.

[0033] According to embodiments consistent with FIG. 1A different band-gap
semiconductor layers are included. The bandgap of GaP at a temperature of
300 K is approximately 2.26 eV, and the bandgap of GaAs at 300 K is
approximately 1.42 eV. In the case of the structure depicted in FIG. 1A,
the higher energy bandgap semiconductor layers (GaP) are placed in the
upper portion of the solar cell. Here, the upper portion of the cell
includes the first material layers encountered by solar radiation as it
impinges on the cell, regardless of the specific cell orientation.
According to FIG. 1A, the lower energy bandgap semiconductor layers
(GaAs) are placed in a lower portion of the solar cell relative to the
incidence of the solar radiation.

[0034] Several parameters are of relevance in the design of a multilayered
stack as depicted in FIG. 1A. The absorption coefficient of the different
layers is important to determine the absorption spectrum of the overall
structure. Furthermore, the index of refraction and thickness of each of
the layers will determine the precise distribution of the optical field
inside the structure, for a given wavelength. This may be obtained
through the same phenomenon giving rise to a photonic bandgap structure,
as described in detail in the paper by M. Scalora et al., "Transparent,
metallo-dielectric, one-dimensional, photonic band-gap structures," J.
Appl. Phys. 83(5), 2377 (1998), incorporated herein by reference in its
entirety. Thus, a configuration may be designed such that for wavelengths
in the visible spectral region optical waves are generated having a
maximum intensity in the semiconductor layers, where absorption occurs.
Using the principles of photonic bandgap structures, a low propagation
speed through the structure for light having a wavelength in the visible
spectrum may increase absorption in the semiconductor layers.

[0035] The electron-hole pairs generated in the semiconductors are
separated by Schottky junctions located at the metal-semiconductor
interfaces and they are collected through an electrode network formed by
the metal layers. The total thickness of the proposed solar cells is of
the order of visible wavelengths (less than one micron) and conversion
efficiencies are relatively independent of incident angle and
polarization. For example, the total thickness of a solar cell structure
as shown in FIG. 1A is about 385 nm, which is below the wavelength range
of visible light. Fabrication techniques for a planar multilayer
structure such as shown in FIG. 1A are well established and relatively
cheap. These characteristics make embodiments consistent with FIG. 1A
ideal candidates for portable, lightweight and flexible solar cell
technology.

[0036] Transmission, reflection and absorption of incident light as a
function of wavelength are useful for the analysis of the performance of
a multilayered structure such as described in embodiments of the present
disclosure. Light impinging on a solar cell may have any angle of
incidence. For example, normal incidence corresponds to light propagating
in a direction perpendicular toperpendicular to the plane of the solar
cell. In normal incidence, the response of the solar cell in terms of
transmission, reflection and absorbance of the light is independent of
the state of polarization. The solar cell presents a plane in which
either of two mutually orthogonal polarization states are equivalent, in
normal incidence. For light impinging on a solar cell at an oblique
angle, an angle other than perpendicular, two polarization states may be
distinguished to characterize solar cell performance. To describe the two
polarization states the normal to the surface at the point of incidence
and the propagation direction define the plane of incidence of radiation.
A p-polarization state has a linear polarization vector included in the
plane of incidence, and an s-polarization state has a linear polarization
vector perpendicular to the plane of incidence. The p-polarization vector
and the s-polarization vector are both perpendicular to the propagation
direction.

[0037] FIG. 1B shows the transmission, reflection, and absorption of light
at normal incidence for a photonic bandgap solar cell according to some
embodiments. For example, a solar cell according to the embodiment in
FIG. 1A produces a transmission, reflection, and absorption curve as
shown in FIG. 1B. A multi-layer stack such as in FIG. 1A absorbs and
converts into electricity up to approximately 60% (corresponding to an
average absorption in semiconductor layers of ˜40%) of the solar
spectrum in the visible range (the (the wavelength range 400 nm-700 nm in
FIG. 1B). Also, an average of approximately 20% of the visible light is
transmitted.

[0038] In other portions of the solar radiation spectrum according to FIG.
1B, the same structure consistent with FIG. 1A reflects approximately 80%
of infrared (IR) light and approximately 50% of ultra-violet (UV) light.
Thus, a solar cell structure as shown in FIG. 1A provides sufficient
transparency in the visible wavelength range to be used for decorative
glass applications, while still generating electricity through
absorption. For example, architectural windows using a solar cell
structure as depicted in FIG. 1A may provide a pleasing esthetic view
while generating electricity.

[0039] Semi-transparent, wide-band metal-semiconductor structures
consistent with the embodiments of FIGS. 1A and 1B, absorbing an average
of 60% (corresponding to an average absorption in semiconductor layers of
˜40%) in the visible range and transmitting the rest of the
incident light can be used effectively as windows or windshield coatings.
The coating may act as an efficient multi junction cell if the metal
layers are properly connected to recover the generated current. The
optical design of this device may be characterized by: i) an efficient
semi-transparent coating or shield sufficiently transparent to allow its
use on windows or windshields (T˜20% across the visible range); ii)
optical field localization inside the stack, enhancing the electron-hole
pair generation rate of the structure. The electron-hole generation rate
competes with the doping of the semiconductors in the determination of
the photocurrent. Higher electron-hole generation rates relax the
requirement for doping the semiconductor layers, thereby simplifying the
overall fabrication process and reducing its cost.

[0040] Embodiments consistent with FIGS. 1A and 1B include seven (7)
layers of silver stacked together with gallium arsenide (GaAs) and
gallium phosphide (GaP) layers to absorb an average of 60% (corresponding
to an average absorption in semiconductor layers of ˜40%) in the
visible range. An absorption maximum reaches approximately 100%
(corresponding to an absorption in semiconductor layers of ˜95%)
near 400 nm, on the `blue` side of the visible spectrum. The different
absorption bands of GaAs and GaP can be efficiently overlapped to produce
the desired transmission, reflection and absorption rates at different
wavelengths. A desirable characteristic of embodiments such as depicted
in FIGS. 1A and 1B is the ability to reflect UV, near-IR (800-1000 nm),
and IR radiation. Thus, avoiding detrimental heating of the cell and
performance deterioration in terms of conversion efficiency, due to
environmental conditions. Moreover, if the structure is used as window
coating, a spectral characteristic as shown in FIG. 1B avoids also the
transmission of IR and UV radiation into an interior environment. Thus
preventing the heating of a room in summertime, and radiation cooling
(radiant heat escaping through windows) in wintertime. The same
phenomenon occurs when this type of coating is employed as a coating on
top of windshields for cars, helicopters or aircraft among other
applications. In addition, a photonic band gap solar cell coating or film
similar to those described above can be applied to mobile computing
devices such as laptops, tablet computers, personal data assistants,
media players, and mobile phones. When applied on a screen of such a
device, the layers are configured to allow at least 25% transmission of
light from the underlying display elements. Such a coating provides UV
and IR protection for the device, while simultaneously generating power
in the solar cell to power the device.

[0041] FIGS. 2A-3C are a series of charts showing transmission,
reflection, and absorption properties as a function of the total amount
of incident light at various wavelengths incident on the PBG structure
over a range of incident angles. Normal incidence corresponds to light
propagating in a direction perpendicular to the plane of the solar cell
shown by 0 degrees with oblique incidence shown extending up to 90
degrees. The scale of the right side of each chart provides an indication
of the amount of light transmitted, reflected or absorbed, respectively,
as a portion of the total light incident on the PBG structure.

[0042]FIG. 2A shows transmission of p-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell,
according to some embodiments. For example, a solar cell according to
embodiments consistent with FIG. 1A produces a transmission surface for
p-polarized light as shown in FIG. 2A.

[0043] FIG. 2B shows the reflection of p-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell,
according to some embodiments. For example, a solar cell according to
embodiments consistent with FIG. 1A produces a reflection surface for
p-polarized light as shown in FIG. 2B.

[0044]FIG. 2c shows the absorption of p-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell,
according to some embodiments. For example, a solar cell according to
embodiments consistent with FIG. 1A produces an absorption surface for
p-polarized light as shown in FIG. 2c.

[0045] FIG. 3A shows the transmission of s-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell,
according to some embodiments. For example, a solar cell according to
embodiments consistent with FIG. 1A produces a transmission surface for
s-polarized light as shown in FIG. 3A.

[0046] FIG. 3B shows the reflection of s-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell,
according to some embodiments. For example, a solar cell according to
embodiments consistent with FIG. 1A produces a transmission surface for
s-polarized light as shown in FIG. 3B.

[0047]FIG. 3c shows the absorption of s-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell,
according to some embodiments. For example, a solar cell according to
embodiments consistent with FIG. 1A produces a transmission surface for
s-polarized light as shown in FIG. 3c.

[0048] The wavelength range covered in FIGS. 2A-2C is from 400 nm to 1000
nm, and the angle of incidence covered is from 0 to 90 degrees, relative
to the normal of the solar cell plane. According to the results shown in
FIGS. 2A-2C and FIGS. 3A-3C, embodiments consistent with FIG. 1A present
optical properties substantially similar for almost all angles of
incidence and for s- and p-polarization states alike. A multilayer
structure such as in the embodiment depicted in FIG. 1A has a total
thickness of the order of about the wavelength of light targeted for
photo-absorption, or less. These structures present substantially similar
optical characteristics for a wide range of wavelengths and angles of
incidence since diffraction effects within the structure are suppressed.
However, since the geometry of the cell is not symmetric with respect to
the normal direction, the ordering of the layers is important to obtain a
desired optical performance. For example, in embodiments consistent with
FIG. 1A, lower energy bandgap semiconductor layers (GaAs) are placed
towards the bottom of the solar cell, away from the point of incidence of
the solar radiation. This gives rise to optical performance as depicted
in FIGS. 1B, 2A-2C, and 3A-3C. For embodiments consistent with FIG. 1A,
light is incident on the GaP side of the solar cell. Note that, according
to FIG. 1B, absorption in the UV portion of the spectrum is relatively
high, thus exploiting the UV portion of the spectrum to generate a
photocurrent. Also according to FIG. 1B, UV radiation is strongly
suppressed in transmission, making a solar cell as in FIG. 1A useful in
applications where UV protection is desirable. Such applications include
eye protection devices, window screens, and others.

[0049] FIG. 4A shows a partial view of a photonic bandgap solar cell,
according to some embodiments. In this embodiment the multilayer is
designed as a wideband solar cell. This structure supports high field
localization in the semiconductor layers favored by the creation of
Fabry-Perot resonances. In order to absorb the widest possible spectrum
the structure includes different types of semiconductors so that the cell
can selectively absorb solar light by exploiting different band-gap
materials. Embodiments consistent with FIG. 4A are obtained by
alternating five (5) silver (Ag) layers each 15nm thick with
semiconductors (GaP, GaAs and Ge) having different band gaps. The
structure has a total thickness of 505 nm. It is worth noting that the
incident side according to embodiments consistent with FIG. 4A is the one
with the higher energy bandgap semiconductor (GaP). The configuration in
FIG. 4A results in wide optical absorption across the visible part of the
spectrum.

[0050] In embodiments consistent with FIG. 4A the different semiconductor
layers are arranged so that high-energy photons (typically UV and visible
light) are absorbed by large-band-gap semiconductors or other
appropriately doped dielectric material on the upper part of the cell. As
solar radiation traverses through a multilayered structure such as shown
in FIG. 4A higher-energy photons (shorter wavelength, at the blue portion
of the spectrum) are absorbed and depleted from the propagating radiation
first. Lower-energy photons (toward the red portion of the spectrum) are
absorbed in small-band-gap semiconductor layers located on the bottom
part of the cell. This arrangement of the semiconductor layers optimizes
the absorption efficiency of the structure. While absorption capacity of
the small-band-gap layers is dedicated to longer wavelengths, shorter
wavelengths are absorbed more efficiently by larger bandgap layers in
early stages of propagation through the solar cell. The result is a
highly absorptive structure having very low transmission, as will be
described below in relation to FIG. 4B.

[0051]FIG. 4B shows the transmission, reflection, and absorption of light
at normal incidence for a photonic bandgap solar cell according to some
embodiments. For example, a solar cell according to embodiments
consistent with FIG. 4A produces a transmission, reflection, and
absorption curve as shown in FIG. 4B. Note that the average transmission
across the visible range at normal incidence is almost 0%, while the
average absorption is ˜90% (corresponding to an average absorption
in semiconductor layers of 70%), and reflection is ˜10%. Moreover
this structure reflects substantially all NIR and IR radiation,
preventing excessive, detrimental heating of the structure. In
embodiments consistent with FIGS. 4A and 4B, the multilayer is designed
to maximize light absorption to an average of approximately 90% of the
visible light, reflecting an average of approximately 80% of IR light and
an average of approximately 50% of UV radiation. This type of embodiment
may be more appropriate for roof-tops or other areas with similar
functionality.

[0052] FIGS. 5A-6C are a series of charts showing transmission,
reflection, and absorption properties as a function of the total amount
of incident light at various wavelengths incident on the PBG structure
over a range of incident angles. The scale of the right side of each
chart provides an indication of the amount of light transmitted,
reflected or absorbed, respectively, as a portion of the total light
incident on the PBG structure.

[0053] FIG. 5A shows transmission of p-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell,
according to some embodiments. For example, a solar cell according to
embodiments consistent with FIG. 4A produces a transmission surface for
p-polarized light as shown in FIG. 5A.

[0054]FIG. 5B shows the reflection of p-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell,
according to some embodiments. For example, a solar cell according to
embodiments consistent with FIG. 4A produces a reflection surface for
p-polarized light as shown in FIG. 5B.

[0055]FIG. 5c shows the absorption of p-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell,
according to some embodiments. For example, a solar cell according to
embodiments consistent with FIG. 4A produces an absorption surface for
p-polarized light as shown in FIG. 5c.

[0056] FIG. 5A shows the transmission of s-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell,
according to some embodiments. For example, a solar cell according to
embodiments consistent with FIG. 4A produces a transmission surface for
s-polarized light as shown in FIG. 5A.

[0057]FIG. 5B shows the reflection of s-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell,
according to some embodiments. For example, a solar cell according to
embodiments consistent with FIG. 4A produces a transmission surface for
s-polarized light as shown in FIG. 5B.

[0058]FIG. 5c shows the absorption of s-polarized light for different
wavelengths and angles of incidence for a photonic bandgap solar cell,
according to some embodiments. For example, a solar cell according to
embodiments consistent with FIG. 4A produces a transmission surface for
s-polarized light as shown in FIG. 5c.

[0059] FIG. 7A shows a partial view of a photonic bandgap solar cell
including SiO2 layers separating metal layers and semiconductor layers,
according to some embodiments. The stack shown in FIG. 7A is similar to
that of FIG. 4A, except for the inclusion of the SiO2 dielectric layers.
The gold (Au) layer has been thinned to approximately 13nm and associated
with a SiO2 layer of approximately 10 nm in thickness. Each of the Ge
layers have been thinned by 10%. Embodiments consistent with FIG. 7A have
good electrical efficiency and comparable optical performance to
structures consistent with FIG. 4A, as described in detail below in
reference to FIG. 7B.

[0060] FIG. 7B shows the transmission, reflection, and absorption of light
at normal incidence for a photonic bandgap cell consistent with FIG. 7A.
FIG. 7B shows a similar optical performance to structures consistent with
FIG. 4A (cf. FIG. 4B). Furthermore, structures consistent with FIG. 7A
have a high conversion efficiency due to the formation of a thicker
depletion region in the semiconductor layer next to the SiO2 layer. In
particular, in a configuration consistent with FIG. 7A the multilayer
film is approximately 515 nm thick and exhibits an optical response
similar to the structure in FIG. 4A (cf. FIG. 7B and FIG. 4B).
Transmission in this case is a bit higher and the average absorption
values are approximately 85% (corresponding to an average absorption in
semiconductor layers of ˜60%) across the entire range disclosed
(cf. FIG. 7B). While the optical response is only slightly affected by
the SiO2 layers, the electrical behavior of the cell benefits from the
presence of insulator layers at each metal-semiconductor junction. With a
SiO2 layer separating a semiconductor from a metal layer, the structure
can sustain higher open-circuit voltages and keep virtually the same
short circuit currents.

[0061] FIG. 8 shows a partial view of an electrical connection for a
photonic bandgap solar cell according to some embodiments. Layers X1
through Xn may be appropriately doped semiconductor layers, and layers Y
may be metal layers. According to embodiments consistent with FIG. 8,
layers X1 through Xn may include different semiconductor materials and
have different thicknesses. Layers Y may include the same metal component
and have the same or different thicknesses. According to embodiments
consistent with FIG. 8 a direct voltage (DC) V may be applied between two
separate metal layers in a stack. For example, the first metal layer may
be placed at a high voltage and the bottom layer may be grounded, as
illustrated in FIG. 8. In such configuration, a potential difference is
established across the stack such that each metal layer in between the
top layer and the bottom layer is at a different voltage value lower than
V and higher than ground. Thus, the metal layers in FIG. 8 are coupled in
series. Each semiconductor layer Xi is placed between two metal layers Y
at different voltages. As a result, when an incoming photon generates
charge carriers such as an electron-hole pair in layer Xi, the positive
charge carriers are separated from the negative charge carriers and
driven into the metal layers from where they may be collected. Some
embodiments consistent with the arrangement of FIG. 8 may have the top
metal layer Y connected to ground, and the bottom metal layer Y connected
to a higher DC voltage V.

[0062] FIG. 9 shows a partial view of an electrical connection for a
photonic bandgap solar cell according to some embodiments. In FIG. 9,
metal layers Y are connected in parallel, by pairs, such that each
semiconductor layer Xi is sandwiched between a metal layer Y at a high DC
voltage V, and a metal layer Y connected to ground. The effect on the
semiconductor layer Xi is the same as described above in relation to FIG.
8. Thus, a photo-generated charged carrier pair within the semiconductor
is split so that charges of opposite sign travel to opposite metal layers
Y, where they are collected away from the stack.